The present invention relates to polymer films and methods for preparing polymer films. Specifically, the present invention relates to bioriented polyethylene films having high water vapor transmission rates (WVTR) and methods of preparing the same.
Generally, in the preparation of a film from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages, including melt film formation, quenching, and windup. For a general description of these and other processes associated with film-making, see K R Osborn and W A Jenkins, Plastic Films: Technology and Packaging Applications, Technomic Publishing Co., Inc., Lancaster, Pa. (1992).
An optional part of the film-making process is a procedure known as xe2x80x9corientation.xe2x80x9d The xe2x80x9corientationxe2x80x9d of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of xe2x80x9corientationxe2x80x9d is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.
Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (Tg) but below its crystalline melting point (Tm), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch, and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength. Further, as a general rule, higher orientation correlates with lower WVTR values for films.
Previously, high WVTR values have been difficult to achieve with polyolefin films. Typically, film production methods aim to lower WVTR values for polyolefin films. As such, polyolefin films inherently have low WVTR values compared to traditional wrapping materials such as cellulose films or paper.
Accordingly, it is one of the purposes of this invention, among others, to produce bioriented polyethylene films having high WVTR values, by providing an economical and relatively uncomplicated method of making polyethylene films that imparts superior characteristics to the films, without requirement for chemical additives such as cross-linking agents, and without requirement for supplemental processing steps such as irradiation of the film.
It has now been discovered that these and other purposes can be achieved by the present invention, which provides for bioriented polyethylene films having high WVTR values and methods of producing the same.
Various structured polyethylene films having high WVTR can be produced by the methods of the present invention. One embodiment of the present invention provides for a film having a base layer and at least one layer of a WVTR controlling material whereby the polyethylene film has a desired WVTR and the base layer has a porous microstructure and WVTR substantially higher than the desired WVTR for the polyethylene film. The base layer includes polyethylene and a cavitating agent, and preferably, the polyethylene is a medium density polyethylene (MDPE) or a high density polyethylene (HDPE). The base layer also has a first side and a second side. Further, the base layer preferably has a thickness of from about 0.5 mil to about 2.0 mil (1 mil=0.001 inch=100 gauge). It should be noted that any thickness value provided herein does not account for additional thickness resulting from cavitation.
The WVTR-controlling layer of the polyethylene film is coextensive with one of the sides of the base layer and includes a WVTR-controlling material, either a MDPE or a HDPE, which should not have a density greater than that of the polyethylene in the base layer. Further, it is preferable that the WVTR-controlling layer have a thickness of from about 0.03 mil (3 gauge) to about 0.15 mil (15 gauge).
A preferred embodiment of the present invention provides for a film having the desired WVTR and having a tie layer interposed between a base layer and a WVTR-controlling layer wherein the tie layer is coextensive with each of the base layer and the WVTR-controlling layer. The base layer, which includes polyethylene and a cavitating agent, has a first side and a second side. Further, the base layer has a porous microstructure and a WVTR substantially higher than the desired WVTR for the polyethylene film. Preferably, the WVTR-controlling layer includes a WVTR-controlling material of either MDPE or HDPE. Alternatively, the WVTR-controlling material can be either an ethylene-propylene copolymer or an ethylene-propylene-butylene terpolymer wherein the tie layer would be either a MDPE or a blend of HDPE and low density polyethylene (LDPE).
Another preferred embodiment of the present invention provides for a film having the desired WVTR wherein the film has first and second layers of a WVTR-controlling material coextensive with the first and second sides of the base layer. The base layer, which includes polyethylene and a cavitating agent, has a porous microstructure and a WVTR substantially higher than the desired WVTR for the polyethylene film. The first and second WVTR-controlling layers should each include a WVTR-controlling material of preferably either a MDPE or a HDPE.
Another preferred embodiment of the present invention provides for a film wherein a first tie layer is interposed between a base layer and a first WVTR-controlling layer and a second tie layer is interposed between the base layer and a second WVTR-controlling layer. The first tie layer is coextensive with each of the base layer and first WVTR-controlling layer and the second tie layer is coextensive with each of the base layer and the second WVTR-controlling layer.
It should be noted that polyethylene films according to the present invention are not limited to the foregoing structures and can include several layers as a plurality of WVTR-controlling layers or tie layers can be provided to obtain a film having the desired WVTR.
The methods of the present invention provide for adherently superimposing at least one layer of a WVTR-controlling material coextensively to a first side of a base layer having first and second sides and then biaxially orienting the composite polyethylene sheet to obtain a film having a desired WVTR. The base layer of the composite polyethylene sheet includes polyethylene and a cavitating agent. The composite polyethylene sheet is biaxially oriented whereby there is provided a bioriented polyethylene film having the desired WVTR and whereby the base layer has a porous microstructure and a WVTR substantially higher than the desired WVTR.
Preferably, the polyethylene of the base layer is a MDPE or a HDPE. Further, it is preferable that the amount of polyethylene provided in the base layer prior to producing a film be an amount sufficient to yield a base layer in the film having a thickness of from about 0.5 mil to about 2.0 mil. It should be noted that any thickness value provided herein does not account for additional thickness resulting from cavitation.
In addition, it is preferable that the WVTR-controlling material is a MDPE or a HDPE, however, the WVTR-controlling material should not have a density greater than that of the polyethylene in the base layer. Further, it is preferable that the WVTR-controlling material is provided in an amount sufficient to yield a WVTR-controlling layer in the film having a thickness of from about 0.03 mil to about 0.15 mil.
A preferred method of the present invention provides for producing a film having a three layer structure. In particular, a first layer of a WVTR-controlling material is adherently superimposed to a first side of a base layer and a second layer of a WVTR-controlling material is adherently superimposed to the second side of the base layer. The composite polyethylene sheet is then biaxially oriented.
Another preferred method of the present invention provides for producing a film having a five layer structure. In particular, a base layer having first and second sides is provided and is interposed between two tie layers wherein the first tie layer is interposed between the first side of the base layer and a first WVTR-controlling layer, and wherein the second tie layer is interposed between the second side of the base layer and a second WVTR-controlling layer. The tie layers are each adherently superimposed to the base layer and are each coextensive with the base layer. The WVTR-controlling layers are each adherently superimposed to the tie layers and are each coextensive with their adjacent tie layer. The composite polyethylene sheet is then biaxially oriented. The tie layers of this structure include MDPE or HDPE and can also include a cavitating agent to regulate the WVTR.
Another method of the present invention provides for producing a film by adherently superimposing at least one tie layer to a base layer wherein the tie layer is coextensive with the base layer, and further, adherently superimposing a WVTR-controlling layer to the tie layer wherein the WVTR-controlling layer is coextensive with the tie layer. The WVTR-controlling layer comprises a WVTR-controlling material of preferably HDPE or MDPE. The tie layer includes MDPE or HDPE and can also include a cavitating agent to regulate the WVTR. However, an alternative method includes provision for a WVTR-controlling layer including a WVTR-controlling material of either an ethylene-propylene copolymer or an ethylene-propylene-butylene terpolymer wherein the tie layer includes either a MDPE or a blend of HDPE and LDPE.
The present invention provides methods of producing polyethylene films having high WVTR values, opacity, high stiffness and resistance to humidity. The films also have excellent deadfold characteristics which make them well suited for packaging of foods in bag-in-box operations conducted on vertical, form, fill and seal (VFFS) machinery. These properties make these films an excellent alternative to paper or cellophane in applications where high WVTR and insensitivity of film to moisture are required.
These and other advantages of the present invention will be appreciated from the detailed description and examples which are set forth herein. The detailed description and examples enhance the understanding of the invention, but are not intended to limit the scope of the invention.